Zeolite materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieve" and are further being used as catalysts or supports for catalysts for the conversion of compounds, for example, in hydrocarbon cracking, alkylation or isomerization reactions.
These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number or various cations, such as Ca, Sr, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selections of the cation.
The remarkable properties of zeolites have led to the development of processes for the production of synthetic zeolites. There are many synthetic zeolites and they form the subject matter of many patents and publications. Non-limiting examples of such synthetic zeolites include A (U.S. Pat. No. 2,882,243), X (U.S. Pat. No. 2,882,244), LZ-210 (U.S. Pat. No. 4,503,023), SSZ-12 (U.S. Pat. No. 4,544,538), SSZ-16 (U.S. Pat. No. 4,508,837, Nu-3 (U.S. Pat. No. 4,372,930).
Generally, crystalline silicate zeolites are synthesized by preparing a solution containing sources of an alkali metal oxide, a nitrogen-containing cations (also known as an organic template), an oxide of aluminum, an oxide of silicon, and water. Then, under well defined operating conditions and with specific ratios between the precursors of the zeolites, crystallizing the desired zeolite.
While crystalline components are generally not good reagents in zeolite synthesis, owing to their resistance to breakdown and transformation, occasionally there are exceptional materials which turn out to be very useful reactants. This may come as a result of such features as very high surface area for the crystalline material, or there may be an unusual inherent instability in the crystal. Very small crystal size can also be a helpful feature.
It was previously shown in U.S. Pat. No. 4,689,207 that a crystalline silica, Magadiite was a useful reactant for high silica zeolite synthesis. U.S. Pat. No. 4,503,024 also discloses a method of preparing chabazite, merlinoite, edingtonite, ZSM-5, and ZSM-11 from the natural and synthetic zeolites mordenite, ferrierite, clinoptilolite, zeolite X and zeolite Y.
For a given zeolite structure it is not always possible to prepare the zeolite over a wide SiO.sub.2 /Al.sub.2 O.sub.3 compositional range. While the zeolite is largely composed of SiO.sub.2, it is the substitution of aluminum for silica which imparts acid characteristics to the zeolites.
It has been demonstrated by Barthomeuf (J. Phys Chem. 1984 pg 42) that the maximum zeolitic acidity is obtained when the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is from 12-18. In general, zeolites with SiO.sub.2 /Al.sub.2 O.sub.3 of 10-20 are catalytically more active than zeolites with values on either side of this ratio. In attempting to modify the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio to a desired range, the prior art teaches several methods for replacing framework aluminum. These methods largely comprise techniques where alumina is removed from the crystalline zeolite structure thereby reducing the acidity of the zeolite. U.S. Pat. No. 5,098,687 discloses a process for removing framework aluminum from zeolites and substituting iron and/or titanium. Other methods include ion exchanging the aluminum using acid or bases or combinations of both methods.
Examples of the above methods include: U.S. Pat. No. 3,620,960 (treatment of the zeolite with molybdenum fluoride); U.S. Pat. No. 3,630,965 (treatment of the zeolite with hydrofluoric acid); U.S. Pat. No. 3,644,220 (treatment of the zeolite with volatile halides selected from the group consisting of aluminum, zirconium, titanium, tin, molybdenum, tungsten, chromium, vanadium, antimony, bismuth, iron, platinum group metals and rare earths); U.S. Pat. No. 3,575,887 and U.S. Pat. No. 3,702,312 (treatment of the zeolite with fluorides and chlorides).
Applicants have now unexpectedly discovered that it is possible to prepare a reaction mixture so that optimum SiO.sub.2 /Al.sub.2 O.sub.3 is incorporated into the zeolite without resorting to subsequent process steps.
It is an object of the invention to provide a method whereby zeolites having SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio less than about may be prepared. It is another object of the invention to prove a method for preparing zeolites whereby crystallization time is substantially reduced as compared to conventional prior art methods utilized. It is still another object of the invention to provide a reproducible method for preparing well crystallized zeolites.
We have discovered that faujasitic structures with SiO.sub.2 /Al.sub.2 O.sub.3 values in the 2 to 20 range, make excellent aluminum sources in zeolite syntheses toward attaining this maximum acidity. In addition these faujasitic materials can often lead to novel zeolites which could not be made when using the methods taught by the prior art.